1. Field of the Invention
The present invention relates to liquid crystal display devices. More particularly it relates to liquid crystal display devices implenting in-plane switching (IPS) where an electric field to be applied to liquid crystals is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That alignment direction can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling an applied electric field, a desired light image can be produced.
Of the different types of known liquid crystal displays (LCDs), active matrix LCDs (AM-LCDs), which have thin film transistors (TFTs) and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
LCD devices have wide application in office automation (OA) equipment and video units because they are light, thin and have low power consumption characteristics. The typical liquid crystal display panel has an upper substrate, a lower substrate and a liquid crystal layer interposed there between. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors and pixel electrodes.
As previously described, LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
In a conventional LCD device, since the pixel and common electrodes are positioned on the lower and upper substrates, respectively, the electric field induced between them is perpendicular to the lower and upper substrates. However, the conventional LCD devices having the longitudinal electric field have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. A detailed explanation about operation modes of a typical IPS-LCD panel will be provided referring to FIG. 1.
FIG. 1 is a schematic cross-sectional view illustrating a concept of a related art IPS-LCD panel. As shown in FIG. 1, upper and lower substrates 10 and 20 are spaced apart from each other, and a liquid crystal layer 30 is interposed there between. The upper and lower substrates 10 and 20 are often referred to as an array substrate and a color filter substrate, respectively. On the lower substrate 20 are a common electrode 22 and a pixel electrode 24. The common and pixel electrodes 22 and 24 are aligned parallel to each other. On a surface of the upper substrate 10, a color filter layer (not shown) is commonly positioned in a position between the pixel electrode 24 and the common electrode 22 of the lower substrate 20. A voltage applied to the common and pixel electrodes 22 and 24 produces an electric field 26 through the liquid crystal 32. The liquid crystal 32 has a positive dielectric anisotropy, and thus it aligns parallel to the electric field 26.
Now the description will illustrate the operation of a related art IPS-LCD device. When no electric field is produced by the common and pixel electrodes 22 and 24, i.e., the off state, the longitudinal axes of the liquid crystal (LC) molecules 32 are parallel and form a definite angle with the common and pixel electrodes 22 and 24. For example, the longitudinal axes of the LC molecules 32 are arranged parallel with both the common and pixel electrodes 22 and 24.
On the contrary, when a voltage is applied to the common and pixel electrodes 22 and 24, i.e., the on state, an in-plane electric field 26 that is parallel to the surface of the lower substrate 20 is produced because the common and pixel electrodes 22 and 24 are on the lower substrate 20. Accordingly, the LC molecules 32 are re-arranged to bring their longitudinal axes into coincidence with the electric field 26.
Therefore, the result is a wide viewing angle that ranges from about 80 to 85 degrees in up-and-down and left-and-right sides from a line vertical to the IPS-LCD panel, for example.
FIG. 2 is a plan view illustrating one pixel of an array substrate according to a related art IPS-LCD device. As shown, gate lines 40 are transversely arranged and data lines 42 are disposed substantially perpendicular to the gate lines 40. A common line 50 is also transversely arranged parallel with the gate line 40 and is spaced apart from the gate line 40. The gate line 40, the common line 50 and a pair of the data lines 42 define a pixel region P on the array substrate. A thin film transistor (TFT) is disposed a corner of the pixel region P near the crossing of the gate and data lines 40 and 42.
In each one pixel P, three common electrodes 44 extend perpendicularly from the common line 50, and two of the common electrodes 44 are disposed next to the data lines 42, respectively. A pixel connecting line 48 is disposed next to the gate line 40 with being parallel with the gate line 40, and is electrically connected to the TFT T. Pixel electrodes 46 extend perpendicularly from the pixel connecting line 48 toward the common line 50. Each of the pixel electrodes 46 is disposed between two of the common electrodes 44 parallel with the data line 42. Each of areas “I” between the respective common electrodes 44 and the respective pixel electrodes 46 is defined as a block where the liquid crystal molecules are re-arranged by the electric fields generated between the common and pixel electrodes 44 and 46. In FIG. 2, there are four blocks in one pixel.
As shown in FIG. 2, the IPS-LCD device according to the related art re-arranges and operates the liquid crystal molecules using the electric field generated parallel with the array substrate. Thus, it can provide a wider viewing angle than the LCD device forming the electric field perpendicular to the array substrate. Some further modifications have been developed in the IPS-LCD device in order to further increase the viewing angle.
FIG. 3 is a plan view of an array substrate for use in an IPS-LCD device having multiple domains according to another related art. With reference to FIG. 3, some of detailed explanations, especially previously explained with reference to FIG. 2, will be omitted in order to prevent duplicate explanations.
In FIG. 3, a pixel connecting line 58 is disposed over a common line 60. Common and pixel electrodes 54 and 56 are elongated from the common and pixel connecting lines 60 and 58, respectively, in an up-and-down direction. Both the common and pixel electrodes 54 and 56 have a zigzag shape with plural bent portions, and they are parallel to each other and arranged alternately. The zigzag shape defines the multidomains in the pixel regions which are symmetrical to the bent portions of the common and pixel electrodes 54 and 56. These zigzag shaped structures and the multidomains improve the viewing angle further that the straight shape of FIG. 2.
Moreover in FIG. 3, the pixel connecting line 58 overlaps the common line 60 so that an overlapped area becomes a storage capacitor CST. Namely, the pixel connecting line 58 acts as one electrode of the storage capacitor CST, while the overlapped portion of the common line 60 acts as the other electrode of the storage capacitor CST. One of the pixel electrodes 56 is connected to a drain electrode 62 so that all of the pixel electrodes 56 can electrically communicate with the TFT T.
However, the IPS-LCD device having the above-mentioned multidomains has a problem in that colors shift depending on the viewing angles, due to the liquid crystal molecules have long and thin shapes.
FIG. 4 is a graph illustrating viewing angle properties of the IPS-LCD device having the zigzag structure of FIG. 3. The IPS-LCD device having the zigzag-shaped common and pixel electrodes can have the improved viewing angles in directions of ±90 and ±180 degrees, i.e., in right-and-left and up-and-down directions, as illustrated by references “IVa” and “IVb” in FIG. 4. However, the viewing angles are degraded in directions of ±45 and ±135 degrees, i.e., in diagonal directions, as illustrated by references “IVc” and “IVd” in FIG. 4. Furthermore, color shift also occurs depending on the viewing angles or directions.
When the voltages applied to the electrodes generate the electric fields between the common and pixel electrodes, the liquid crystal molecules rotate about 45 degrees in accordance with the electric fields. Gray inversion occurs due to the rotation of the liquid crystal molecules. In particular, when the IPS-LCD is operated in gray mode, the IPS-LCD produces yellowish color in 45(+45) degrees declination with respect to the liquid crystal polarization because of the optical anisotropy properties of liquid crystal molecules. And the IPS-LCD also produces bluish color in 135(−45) degrees declination with respect to the liquid crystal polarization because of the optical anisotropy properties of the liquid crystal molecules.